I. Field of the Invention
This invention relates to apparatus for measuring the cross-sectional parameters of a light spot and, more particularly, to such apparatus employing knife-edge techniques for measuring the size and intensity profile of a focused laser light spot in both of two orthogonal directions thereof.
II. Description of the Prior Art
A well known technique for measuring the size and intensity profile of a light spot employs an opaque member having a knife-edge which is moved through a cross section of a light beam. A light-power sensor, such as a photocell, is positioned on the other side of the opaque member from the incident light beam to intercept substantially all of the occulted light. The photocell signal is a measure of the occulted light power as a function of knife-edge position and the first derivative of this photocell signal is a measure of the occulted light intensity as a function of knife-edge position.
In the past, an accurate measurement required accurate knowledge of the knife-edge displacement. By way of example, the knife-edge can be displaced with a scanner comprised of a piezo-electric translator and this displacement can be monitored with an interferometer. In order to make a two-dimensional measurement of the spot profile, it was necessary to rotate the knife-edge scanner through ninety degrees, and repeat the measurements. This limited the accuracy of the measurement, since it is not possible to maintain a constant optical-axis (z-axis) position to sub-micrometer tolerances.
More recently, a simultaneous two-dimensional measurement has been demonstrated, using two orthogonal knife edges on a scanning "fishtail" shaped mask. The mask is translated using a synchronous motor with an additional optical encoder for calibration purposes. An alternative method for calibration employs a one-dimensional Ronchi ruling of known dimensions as the knife-edge mask in order to obtain a signal with features from which the speed of the mask translation can be determined.